Piston-cylinder assemblies of this kind are known and depending on their design, are referred to as differential cylinders or synchronizing cylinders. They are essentially used in hydraulic and pneumatic applications in order to transmit forces in all conceivable working directions. Particularly in the context of the present invention, such piston-cylinder assemblies can be used for presses. The term “press” is understood in this context to be a generic term for variously functioning hydraulic presses, by means of which, through the hydraulic exertion of force, an extremely wide variety of products can be shaped or manufactured. Examples of such presses include a hydraulic stamping press, guillotine shears, a press for the fireproofing and tile industry, a press for manufacturing salt products, etc.
The shaping process for products can be performed in such a way that two main axes, at least one of which is a movable axis, are moved in relation to each other and thus execute the shaping procedure. In a press used in the fireproofing industry, loose bulk material, for example, is pressed into a mold by the relative movement of the main axes, which at least partially establishes the shape of the pressed item manufactured by means of the pressing procedure. By contrast with a stamping press process or guillotine shears process, the end of which is established by the completion of the stamping step or shearing procedure, with the above-described press used in the fireproofing industry, the shaping procedure is discontinued when the axes have traveled a certain distance, when a certain pressure has been reached in the main cylinders, or when both of these criteria lie within a defined tolerance range.
Piston-cylinder assemblies of the type mentioned at the beginning are not used exclusively for the main cylinders or main working axes; there can also be auxiliary functions for which the piston-cylinder assemblies are likewise used. One such auxiliary function, for example, is the movement of the mold wall of the mold after completion of the pressing procedure in an above-introduced press from the domain of the fireproofing industry. This is the so-called demolding of the pressed item from the mold; the pressed item rests against a stationary die or main cylinder while the mold wall is moved relative to the main working axis by means of a movement produced by a piston-cylinder assembly, thus removing the mold from the pressed item.
Depending on the arrangement of the auxiliary cylinder in relation to the press, the demolding procedure can occur as a result of an effective direction oriented in the direction of the extending or retracting piston rod of the auxiliary cylinder. Naturally, if the mold wall is kept stationary, it is also possible to demold the pressed item through a movement of a main cylinder.
However, it has turned out that piston-cylinder assemblies of the type mentioned at the beginning are only satisfactory to a limited degree with regard to their durability in the normally customary technical design since after a relatively short period of operation, damage occurs in the cylinders themselves, e.g., the welded seams, and in other components coupled to the piston-cylinder assemblies, such as position measuring systems or line systems; various other mechanical damages occur as well. The observed, less-than-satisfactory service life of the piston-cylinder assembly and other components means that the corresponding parts have to be embodied in a reinforced way since otherwise, the parts have to be repaired or replaced, which is expensive, and where necessary, the press may not be operational during repair work, thus resulting in production downtimes.
Attempts have been made to remedy this problem by building dampers such as hydropneumatic shock absorbers into the line systems to which the piston-cylinder assemblies are connected. Such measures, however, have not had the desired effect.
U.S. Pat. No. 2,815,004 has disclosed pneumatic pruning shears in which the recoil of a piston situated in a cylinder is cushioned by the combined action of an air cushion provided in the cylinder and a spring device attached to the piston.
GB 1 563 847 has disclosed an apparatus for producing an instantaneous pressure on a workpiece. The apparatus disclosed therein includes a piston-cylinder assembly in which a subchamber of the cylinder interior oriented away from the workpiece is filled with a gas, which, through an instantaneous volume expansion, is able to exert an instantaneous pressure on the workpiece by means of the piston of the device. This instantaneous expansion is possible in that a preceding pressure-relief valve produces an instantaneous pressure decrease in the subchamber of the cylinder on the opposite side from the gas-filled subchamber.
EP 0 186 002 A1 has disclosed a device for suppressing pressure peaks in the press assembly of a die-casting machine. In the die-casting machine, a press cylinder with a press piston is provided; inside the press piston, an auxiliary piston is provided, which is movable in the axial direction of the press piston. During an extending movement of the press piston in a mold-filling phase, a braking of the press piston at the beginning of the secondary pressing phase generates a pressure increase in the hydraulic fluid contained in the press cylinder on the piston surface side. The pressure peaks thus occurring at the transition to the secondary pressure phase are prevented by a movement of the auxiliary piston inserted into the press piston. To that end, a connecting line is routed through the press piston to a cylindrical guide situated behind the auxiliary piston.
U.S. Pat. No. 3,536,128 has disclosed an injection assembly for a die-casting machine in which an injection piston is coupled to a hydraulic die via a fluid-filled damping chamber. The damping chamber is situated in a cylinder between the injection piston and the hydraulic die. In a first phase, these two pistons both move at the same speed toward the mold. Upon coming into contact with the filled mold, the injection piston meets a resistance that brakes its movement toward the mold. This results in a pressure increase in the damping chamber that is counteracted by the fact that the fluid contained in the damping chamber is permitted to exit the damping chamber, whose volume decreases in the process. The fluid can escape by means of devices that are situated either inside the hydraulic die or inside the injection piston.